Old thread, but really doesn't have a solution, so here's a short guide, starting from things required:
- PSP with any modern CFW(for example I use 6.60 pro-c fix 3),
- PRXdecrypter 2.7a or any other decrypting homebrew that works on your CFW,
- PBP Unpacker (windows software, no idea if it works through wine), there should be some other programs which can be used instead,
- homebrew we want to decrypt(generally any homebrew that works on PSP, but doesn't even try to boot under emulator showing missing key / corrupted message).

Having all that, it's really simple - start by using PBP Unpacker to extract all files from EBOOT.PBP to a separate folder. Now connect PSP and copy our extracted DATA.PSP into "ms0:/enc/" (if that folder doesn't exist, create it).
Then we run PRXdecrypter homebrew and pick Decrypt/decompress files and wait until it finishes, once it's done just exit the app, connect to PC again and copy your now decrypted DATA.PSP(unless set otherwise ms0:/enc/ is both input and output folder for PRXdecrypter).
Then use PBP Unpacker again to create new EBOOT.PBP using all the same files that was extracted except replacing old DATA.PSP with the decrypted one.

With that game should boot, or at least have a chance to try:].
Reminder - homebrew are a special case and should be ran from inside memstick/PSP/GAME/ folder, some homebrew might also render graphics in ways that will only show with software rendering, not much really can be done about it:].

Hopefully due to PSV which refreshed PSP homebrew scene a bit, most still actively developed homebrew will almost surely have an unsigned version since all PSP emulators are missing same keys. On a side note this should also answer any questions if PSV or any other device that can run decrypting software for PSP stuff can be used - no:].